Japanese carmaker has lost half of its market value – $3.9bn – since it said fuel-economy readings were being falsified.

The Japanese carmaker Mitsubishi Motors has admitted using fuel-economy testing methods that did not comply with Japanese regulations for 25 years – much longer than previously known.

It said on Tuesday that aggressive internal targets may have put pressure on employees to overstate the fuel economy of its vehicles – adding that it would set up an external committee to investigate the matter.

Japan’s sixth-largest carmaker has lost half its market value – some $3.9bn (£2.7bn) – since it admitted last week to manipulating test data for four domestic mini-vehicle models – including two it produced for Nissan.

It has also said that more models may have used tests that did not comply with Japanese standards – prompting concern about ballooning potential compensation costs and fines.

The US vehicle safety regulator is also seeking information – while Japanese authorities have raided one of the company’s research and development facilities.